Despite the title of
Darwinís famous book, we cannot be sure to what extent
natural selection acting on gradual accumulation of small
mutations has accounted for the development of new species.
Many of the examples that are often quoted as demonstrating
that this is the main source of new species show that new
species have arisen in new niches, e.g. by geographic
isolation as in the case of the Galapagos island tortoises
and finches. But this evidence, by itself, does not
establish what the mechanism may have been. Other
interpretations would also be possible.

Darwinís idea of
natural selection was in part based on observing the effects
of artificial selection, producing many different varieties
of dogs, cats, fish etc. But it is important to note that
thousands of years of this kind of selection has produced
new varieties, not fully-fledged new species as defined by
inability to interbreed. At best, this demonstrates
selectionís ability to produce
incipient
development of new species.

North of the Himalayas there are two varieties (subspecies?)
that co-exist but do not interbreed. But they are each
connected to varieties with which they do interbreed
stretching around the Himalayas to join the ring together in
the south. Ticehurst proposed in 1938 that the greenish
warblers started in the south, then slowly evolved as they
spread around the Himalayas to the west and east, to
eventually meet in the north when they had become different
species. Genetic data are consistent with his hypothesis.
This is perhaps the best example we have of this process,
which itself is rare. Note however that this evidence
supports Ticehurstís hypothesis about the historical
development, since the two northern forms are the most
distinct genetically, but it does not in itself prove the
precise mechanism. It does not, for example, exclude a
Waddington-type influence of the environment on the
selection of combinations of genes, or any other process by
which a Lamarckian process becomes assimilated into the
genome. Nor does it exclude natural genetic engineering.
This is a difficulty with any study of existing genetic
variations to infer the process by which those variations
originally arose. The role of natural genetic engineering
could be determined when we have genome sequences from most
or all of the varieties.

By contrast, hybridisation in plants readily generates new
species, and the process of symbiogenesis must also have
done so.Natural
genetic engineering could also do so if it involves DNA
shuffling that removes reproductive compatibility.

The important message here is not that the generation of new
species by natural selection working on random mutations is
impossible, but rather that the examples often quoted to
prove that it happened are not so clear cut when they are
examined closely. We have to distinguish evidence about the
historical
development of new species from evidence about the precise
mechanisms by
which this happened.